Environmental Science & Engineering - www.esemag.com - March 2003
Comments? send them to the editor.
Confused about Best Management Practice performance?
by Todd Neff, P.Eng.,
Stormceptor Canada Inc.
For years hydrologists have been
tackling the challenge of using
flows measured in one catchment
to estimate flows in
another. Similar challenges exist
for stormwater quality treatment
designers when using historical
monitoring data to estimate
future pollutant loads and performance
of stormwater best
management practices (BMPs).
Unfortunately, measured performance
(the efficiency of pollutant
removal) cannot be easily
compared between different
monitoring sites or used to size
future systems.
Generalizations about the
effectiveness of BMPs are often
made from a limited review of monitoring
data. To obtain approval in many
jurisdictions, structural BMPs must
achieve sediment removal efficiency
targets of 80% or more for sensitive
sites (see references). When monitoring
data are reviewed and results show
removal efficiencies were less than
80%, designers and reviewers often
incorrectly conclude that the BMP is
not capable of achieving the efficiency
target.
Consideration must be given to
other factors that contribute to the
measured efficiency. These factors
can increase or decrease perceived performance.
One factor that is most
often ignored is the designed removal
efficiency. If the BMP was designed
to achieve an efficiency of 70%, a
measured efficiency of less than 80%
can still support the conclusion that the
BMP is very effective. Conversely, in
some instances where minimal effort
has been expended to obtain data of
only poor quality, positive conclusions
are erroneously drawn. For example,
grab samples taken upstream and
downstream of a BMP at one instant in
time do not support the conclusion that
the unit is effective over the long term.
Inappropriate conclusions have led
to confusion regarding the effectiveness
of both traditional and proprietary
BMPs. To use field data as a tool for
assessing the effectiveness of future
BMPs, a frame of reference must be
developed, a frame of reference that
effectively normalizes the data. A
number of factors that need to be normalized
and considered when using
test data to estimate future performance
include:
Design – compare the expected performance
to the measured performance.
How well was the measured performance
predicted?
Site and Land Use – consider the
land use at the test site compared to the
design site. The performance of a unit
tested in a public works yard or construction
site will differ from a shopping
mall parking lot, just as the
hydrology and runoff coefficients/
characteristics will differ.
Configuration – consider variations
caused by hydraulic structures, flow
controls, upstream storage or pre-treatment,
use of bypasses (internal or
external), and the use of oil absorbents
in the test unit. The design unit must
have the same configuration as the test
unit to have similar performance.
Monitoring Protocol and Test
Procedures – understand the definition
of the pollutant measured during the
test because a similar name might exist
for a different pollutant or test procedure.
For example, sediment removal
efficiency is highly dependent on sediment
size. High removal efficiencies
can occur at a test site dominated by
coarse sediment. If these results are
used to size a unit on a site with very
fine sediment the design will fail
because the same level of performance
will not be achieved.
Other considerations exist such as
the method of calculating performance
and the meteorological conditions
under which the performance data
were collected.
Before using performance
data or sizing methods based on
these performance data to design
future BMPs, the conditions
under which the measurements
were made must be examined to
determine if they are relevant and
similar to the design site.
Designers should be aware of
dissimilarities between test sites
and design sites and use some
method for normalizing the variations,
such as a calibrated and
validated computer model.
References
Calgary, City of, Stormwater
Management & Design Manual, Draft
2000, p 4-14, 2000.
MDE, Maryland Department of
Environment, Maryland Stormwater
Design Manual Volumes I & II,
September 1998 Review Draft, p1.13,
1998.
MA DEP, Massachusetts Department
of Environmental Protection, and
Massachusetts Office of Coastal Zone
Management, Stormwater Management,
Volume One: Stormwater Policy
Handbook, p1-4, 1997.
ON MOEE, Ontario Ministry of
Environment and Energy, Stormwater
Management Practices Planning and
Design Manual, pp 171-176, 1994.
See our home page on how to order your subscription. We regret we can
only accept orders from Canada and the United States.
